This invention relates to a fuel delivery system for supplying fuel to the combustor of a gas turbine engine and, more particularly, to apparatus for protecting the components of the system from the effects of pressure perturbations occurring within the combustor.
Some modern-day gas turbine engines are being adapted to operate on either gas or liquid fuels or combinations of both. Such capability provides the engine with the versatility to utilize the most economical fuel available at the specific geographical location of the gas turbine engine. Since many of these engines were originally designed for operation solely on liquid fuels, in order to minimize conversion costs, the adaption required to provide dual fuel capability has been accomplished within the design constraints associated with the liquid systems. By way of example, the gas fuel system must be compatible with available supply pressure pump apparatus and the fuel delivery schedule associated with the existing fuel control valve.
One particular problem associated with introducing gas flow operation compatible with liquid fuel system design constraints is gas flow dynamic instability and resonance caused by pressure perturbations occurring within the gas turbine engine combustor. This problem is particularly acute at low flow rates of gaseous fuel into the combustor. More specifically, energy released by the burning fuel within the combustor causes pressure waves or pressure perturbations which effect subsequent delivery of gaseous fuel into the combustor. At low flow rates the gaseous fuel within the fuel delivery system is at a relatively low pressure which is close in magnitude to the nominal pressure within the combustor. The pressure perturbations within the combustor in some instances create transient pressures higher than the pressure of the gaseous fuel within the delivery system. The pressure waves momentarily interupt the flow of gaseous fuel into the combustor by compressing and storing the gaseous fuel within the delivery system. It has been found that the gaseous fuel is compressed as far upstream as the gaseous fuel manifold which, having substantial volume, can store a significant amount of gaseous fuel at an abnormally high pressure. When the pressure perturbation ceases due to the fuel flow stoppage, the compressed gaseous fuel within the delivery system is at a pressure substantially higher than the nominal pressure in the combustor and hence the gaseous fuel is released into the combustor at a momentarily high flow rate which, in turn, subsequently causes another pressure perturbation to occur within the combustor. This process is repeated again and again at a high rate of repetition causing a phenomenon known as dynamic instability, resonance or chugging which may result in substantial damage to the components of the gas turbine engine.
The above-described phenomenon does not occur with liquid fuel systems since liquids are nearly incompressible nor does the phenomenon occur with gaseous fuels under high flow rate conditions since the pressure of the gas within the fuel delivery systems is higher in magnitude than the magnitude of the pressure perturbations occurring within the combustor. The present invention is addressed to overcoming the effect of pressure perturbations within the combustor in a gaseous fuel delivery system under low flow rate conditions. Such conditions can occur when a dual fuel gas turbine engine is operated with a combination of liquid and gaseous fuels simultaneously, or when operating on gaseous fuels at low engine power.